The present invention generally relates to undervoltage load shedding systems for alleviating or preventing system-wide power system voltage collapses and, more specifically, to a time undervoltage load shedding system having a compensated input inverse-time undervoltage element.
Electric utility companies are responsible for maintaining voltage and frequency stability for the area under their control. To achieve this stability, the power utility companies are designed with apparatus and procedures that react to unexpected occurrences of increased or decreased power system voltage levels. Such apparatus and procedures are used to maintain frequency and voltage excursions within acceptable limits under both normal and abnormal operating conditions, without exceeding the thermal limits of the power system components.
Events that contribute to voltage instability of the power system may include, for example, a VAR (voltage-amperes reactive) generating source reaching its operability limits, the loss of a VAR generating source, a decrease of effective transformer turns ratios, a heavy power system loading that approaches or exceeds the power system's limits, and/or tripping of transmission lines and/or generators. In most cases, however, a lone event does not lead to power system voltage collapse. Instead, a sequence of events typically causes the voltage instability which may cascade into localized or widespread system voltage collapse. Accordingly, a voltage collapse may be defined as a process by which voltage instability leads to post-disturbance equilibrium voltages below acceptable limits across one or more portions of a power system or grid.
A primary means of maintaining power system voltage stability includes ensuring that sufficient reactive power reserves are available to maintain nominal power system voltage. Other means include raising a point-of-use voltage to reduce the magnitude of a load current induced voltage drop, increasing generator(s)output voltage via generator controls, and/or decreasing one or more loads served by the power system via tripping the load(s) off-line (i.e., temporarily disconnecting the load(s) from the power system). Tripping a load off-line in response to a power system undervoltage condition is herein referred to as load shedding.
Prior to load shedding, the power system may attempt to adjust an undervoltage condition using first stage corrective actions such as energizing shunt capacitors at or near load centers, activating synchronous condensers, energizing previously de-energized lines from a single source, and/or placing VAR generation sources on-line. Moreover, automatic controls such as load-tap changers (LTCs), placed on the load distribution side of a power transformer of the power system, may be used to dynamically change load transformer turn ratios during periods of voltage instability.
Despite such first stage corrective actions, voltage instability may continue until power system voltage collapse and/or rotor angle instability (i.e., generator loss of synchronism). As a result, load shedding may be necessary.
The simplest undervoltage load-shedding (UVLS) schemes include undervoltage detectors, referred to as undervoltage relays, that act to trip a load off-line (i.e., shed the load from the power system) some predetermined time period after the measured system voltage falls below a preselected voltage level, or threshold. If the measured system voltage does not remain below the pre-selected voltage threshold for longer than the predetermined time period, the undervoltage relay does not shed the associated load. This method of undervoltage load shedding is hereafter referred to as definite time delay undervoltage load shedding.
While effective in some cases, utilizing definite time undervoltage relays for load shedding purposes has several drawbacks. First, because each definite time undervoltage relay includes a timer having one predetermined time period, or interval, and one preselected voltage threshold, loads may be unnecessarily shed. For example, if two identical definite time undervoltage relays having a 2 second preset time interval and a 0.9 per unit nominal voltage magnitude threshold are each coupled to a power circuit breaker associated with a different load, and the power system voltage measured by each time undervoltage relay drops below the 0.9 per unit nominal voltage for 2 seconds, the respective definite time undervoltage relays will cause the coupled power circuit breakers to shed their associated loads, regardless of the load's proportionate contribution to the undervoltage condition. In that case, it may have only been necessary to shed the load drawing the most reactive power rather than shedding both loads.
In other words, any and all loads associated with definite time undervoltage relays meeting the preset time and voltage criteria will be shed. As a result, dynamic loads such as induction motors, load-tap changers and thermostatic loads demanding more reactive power from the system are not necessarily shed before static loads demanding less reactive power from the system. A better load shedding scheme would shed just enough loads to ensure that the power system recovers to its nominal operating voltage.
To reduce the likelihood that a particular load will be shed unnecessarily, undervoltage load shedding elements that do not rely on predetermined time period delays have been utilized. Such undervoltage load shedding elements, hereinafter referred to as inverse time undervoltage load shedding relays, calculate a tripping time, or load shedding time delay, that is a function of measured power system voltage at a particular bus. As a result, the load shedding time delay can vary.
For example, an inverse time undervoltage relay such as an IAV54E Undervoltage Relay manufactured by General Electric Co. may be configured to cause a load to be shed when the measured power system voltage drops below 0.85 per unit nominal voltage for longer than 17 seconds. However, if the measured voltage drops below 0.80 per unit nominal voltage, the inverse time undervoltage relay will act to cause the load to shed in about 14 seconds. Thus, as the power system voltage measured by the relay decreases so does the load shedding time delay.
Although less likely to unnecessarily shed a load than the definite time undervoltage relay, the inverse time undervoltage relay may still unnecessarily shed a load if, for example, power system recovery to nominal operating voltage is slow. An ideal load shedding system would identify the loads demanding the most reactive power and shed those loads first, thereby minimizing the number of loads that are shed until the power system voltage recovers to its nominal operating voltage level.
Therefore, it is an aspect and object of this invention to provide a load shedding system which avoids one or more of the drawbacks of prior load shedding systems.